Fun fact, the (rough) conversion efficiency of calories to mechanical joules in the human body (separate from the mechanical to electrical you're referring to) is about 25%
but this is about the same factor as going from calories to joules! So, for a human to put out 13.5 kJ of energy would require about 13.5 food calories (kilocalories).
My headcanon for The Matrix's "humans are batteries" is that it's the machines' perverse interpretation of this
killing the humans is off the table, and for whatever reason letting them live with no purpose to serve the machines is also disallowed. But giving their lives "meaning" in the form of a shitty (and thermodynamically dubious) "battery" somehow satisfies the rules.
It's a very big stretch, I'll admit...